1. Field of the Disclosure
Embodiments disclosed herein relate generally to cutting structures for use on drilling tool assemblies. More specifically, embodiments disclosed herein relate to asymmetric cutting structures disposed on downhole reamer cutter blocks.
2. Background Art
FIG. 1A shows one example of a conventional drilling system for drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drilling tool assembly 12 that extends downward into a well bore 14. The drilling tool assembly 12 includes a drillstring 16, and a bottomhole assembly (BHA) 18, which is attached to the distal end of the drillstring 16. The “distal end” of the drillstring is the end furthest from the drilling rig. The drillstring 16 includes several joints of drill pipe 16a connected end to end through tool joints 16b. The drillstring 16 is used to transmit drilling fluid (through a central bore) and to transmit rotational power from the drilling rig 10 to the BHA 18. In some cases the drillstring 16 further includes additional components such as subs, pup joints, etc.
The BHA 18 includes at least a drill bit 20. Typical BHA's may also include additional components attached between the drillstring 16 and the drill bit 20. Examples of additional BHA components include drill collars, stabilizers, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, subs, hole enlargement devices (e.g., hole openers and reamers), jars, accelerators, thrusters, downhole motors, and rotary steerable systems. In certain BHA designs, the BHA may include a drill bit 20 or at least one secondary cutting structure or both. In general, drilling tool assemblies 12 may include other drilling components and accessories, such as special valves, kelly cocks, blowout preventers, and safety valves. Additional components included in a drilling tool assembly 12 may be considered a part of the drillstring 16 or a part of the BHA 18 depending on their locations in the drilling tool assembly 12. The drill bit 20 in the BHA 18 may be any type of drill bit suitable for drilling earth formation. Two common types of drill bits used for drilling earth formations are fixed-cutter (or fixed-head) bits and roller cone bits.
In the drilling of oil and gas wells, concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. Each new casing string is supported within the previously installed casing string, thereby limiting the annular area available for the cementing operation. Further, as successively smaller diameter casing strings are suspended, the flow area for the production of oil and gas is reduced. Therefore, to increase the annular space for the cementing operation, and to increase the production flow area, it is often desirable to enlarge the borehole below the terminal end of the previously cased borehole. By enlarging the borehole, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Accordingly, by enlarging the borehole below the previously cased borehole, the bottom of the formation may be reached with comparatively larger diameter casing, thereby providing more flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through an existing cased borehole and enlarging the borehole below the casing. One such method is the use of an underreamer, which has basically two operative states—a closed or collapsed state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased borehole, and an open or partly expanded state, where one or more expandable arms with cutting elements on the ends thereof extend from the tool body. In the expanded position, the underreamer enlarges the borehole diameter as the tool is rotated and lowered in the borehole.
Underreamers with expandable cutter blocks having cutting elements thereon allow a drilling operator to run the underreamer to a desired depth within a borehole, actuate the underreamer from a collapsed position to an expanded position, and enlarge a borehole to a desired diameter. Cutting elements of expandable underreamers may allow for underreaming, stabilizing, or backreaming, depending on the position and orientation of the cutting elements on the blades. Such underreaming may thereby enlarge a borehole by 15-40%, or greater, depending on the application and the specific underreamer design.
Typically, expandable underreamer design includes placing two blades in groups, referred to as a block, around a tubular body of the tool. A first blade, referred to as a leading blade absorbs a majority of the load, the leading load, as the tool contacts the formation. A second blade, referred to as a trailing blade, and positioned rotationally behind the leading blade on the tubular body then absorbs a trailing load, which is less than the leading load. Thus, the cutting elements of the leading blade traditionally bear a majority of the load, while cutting elements of the trailing blade only absorb a majority of the load after failure of the cutting elements of the leading blade. Such design principles, resulting in unbalanced load conditions on adjacent blades, often result in premature failure of cutting elements, blades, and subsequently, the underreamer.
Conventional expandable reamers may be characterized as “near symmetrical,” in that the layout of cutting elements on the multiple cutter blocks is similar and the cutter blocks are equally spaced around a circumference of the underreamer. For example, conventional underreamers may have three cutter blocks spaced 120 degrees apart from each other. Further, each cutter block may have multiple rows of cutting elements thereon, each row having an equal number of cutting elements. Thus, the conventional cutting structure layouts are inherently symmetrical or near symmetrical. While near-symmetrical reamers may be sufficiently stable in a static state (i.e., not moving), variable factors such as changing formation properties, deviated well profiles (e.g., vertical and/or horizontal wells), and variable drilling parameters (e.g., drillstring revolutions per minute, weight on bit, etc.) may cause instability in the reamer when in a dynamic state (i.e., while drilling). In particular, vibrations may be created in the reamer due to the variable factors above. The vibrations may be periodic in nature because of the near symmetrical arrangement of the cutting elements and cutter blocks on the reamer. The vibrations may continue to amplify with each rotation of the reamer unless the pattern is interrupted in some manner.
Accordingly, there exists a need for apparatuses and methods of designing cutting structures for reamers that are capable of interrupting and reducing vibrations created during drilling.